This invention relates to a demultiplexer for a pair of n-channel digroups (digital groups) that are synchronously multiplexed for transmission purposes.
In recent years, the price of telephone cable has been rising sharply because of the increased cost of copper and petroleum-based plastic. Moreover, in rapidly growing areas of the telephone plant, such as the exchange area, the cost of installing new cables has increased significantly. Furthermore, telephone companies are faced with dwindling duct and manhole space. Clearly, one solution to these problems is to put as much call-carrying capacity as possible into a single pair of wires by using carrier facilities.
The T1 carrier system introduced in 1962 (see "The T1 Carrier System", Bell Laboratories Record, Vol. 40, November 1962 by D. F. Hoth, pp. 358-363) was very successful in this regard. T1, by employing pulse code modulation (PCM), encodes 24 voice channels into a 1.544 megabit-per-second digroup signal for transmission over two cable pairs (one for each direction of transmission).
A later digital carrier system, called T1C, has since been developed. T1C has twice the capacity of T1, or 48 high-quality voice channels over two cable pairs and a correspondingly higher line rate of 3.152 megabits-per-second. The great advantage of T1C is of course its increased voice channel capacity, which makes more efficient use of cables, ducts, and manholes; see the article "T1C Carrier: The T1 Doubler", Bell Laboratories Record, June 1975, by James F. Graczyk et al, pp. 257-263.
Two asynchronous T1 signals are combined into a T1C signal by bit interleaving the two. This transmission mode necessitates the use of a "pulse stuffing" technique, as well as added framing information (i.e., superframe bits) to separate the composite T1C signal into its two component parts at the receiving terminal.
Whenever possible, it would appear advantageous to synchronously multiplex the two (24-channel) digroup signals. The electronic circuitry associated with "pulse stuffing" can thence be dispensed with, a common, transmitting terminal, clock source can be used, etc. Conventional practice, however, would still seem to suggest the use of additional framing bits in the composite multiplex signal to permit the same to be separated into its component parts at the receiving terminal.